Bidirectional Forwarding Detection

Bidirectional Forwarding Detection (BFD) is used to detect faults between two routers across a link, even if the physical link does not support failure detection. Even in cases where physical link issues occur and are detected, BFD can coordinate reaction to these failures rather than each component relying on its own failure detection methods.

TNSR uses UDP as a transport for BFD between directly connected routers (single hop/next hop) as described in RFC 5880 and RFC 5881.

Each BFD session monitors one link. Multiple BFD sessions are necessary to detect faults on multiple links. BFD sessions must be manually configured between endpoints as there is no method for automated discovery.


The BFD implementation on TNSR only supports single hop BFD session in the dataplane. As such, BFD can only be configured on directly connected interfaces, between directly connected peers.

BFD supports session authentication using SHA1 for security, and the best practice is to use authentication when possible.

When using BFD, both endpoints transmit “Hello” packets back and forth between each other. If these packets are not received within the expected time frame, with the expected authentication information, the link is considered down. Links may also be administratively configured as down, and will not recover until manually changed.

TNSR currently supports BFD integration with BGP, OSPF, and OSPF6.